Electrolytic surface treatment of metals



United States Patent U.S. Cl. 204-56 7 Claims ABSTRACT OF THE DISCLOSURE In an electrolytic surface treatment of metal, a metal product is used as a cathode in an electrolyte prepared by adding a small amount of water-soluble organic compound containing a thio radical to an aqueous solution of a chromium compound in which the chromium is predominantly present in the solution in the form of hexavalent chromium-containing ion. The water-soluble organic compound containing a thio radical used in this process includes thioformamide, thiourea, thiosemicarbazide, thioglycol, thioacetate, and thiosalicylate. As a result of the treatment, the surface of metal product can be provided with a film of good quality having corrosion resistance.

This invention relates to a method for the electrolytic surface treatment of metal product which comprises subjecting a surface of metal to electrolytic formation treatment. More particularly, it relates to the above-mentioned method wherein a treatment solution prepared by adding a water-soluble organic compound containing a thio radical to an aqueous solution of chromium compound in which the chromium is predominantly present in the solution in the form of hexavalent chromium-containing ions is used as an electrolyte, the metal product is electrolyzed as a cathode and the electrolytic rteatment is practiced.

There have heretofore been various methods in which a metal is subjected to electrolytic formation treatment to provide a film thereon in aqueous solution of chromium compound as described above. In general, these methods are widely known as the chromate treatment method. That is, a process of this kind has been introduced as a process for providing magnesium and its alloys with a corrosion resistant film thereon, and the process has been called Cronak method, which is a registered trademark. Thereafter, the process has been applied to zinc, cadmium, and aluminum. Unichrome method for zinc and cadmium, Alodine method for aluminum and other methods have been extensively developed. Each of Unichrome and Alodine is also a registered trademark. Although there may be observed some diiierences among these methods, they were essentially the methods which comprise electrolytically forming a corrosion resistant film on the surface of metal by merely dipping the metal to be treated in a treatment solution prepared by adding a small amount of acids to an aqueous solution of chromium compound, such as chromic acid.

The composition of such a chromate film electrolytically formed on the surface of metal is represented by the general formula (CrO (Cr O (H O)z, and is said to be a gluey complex compound consisting of a hydrate of trivalent chromium oxide and hexavalent chromium oxide. At the time of treatment, in case a metal product acts with an acidic treatment solution to generate hydrogen and is eluted as metal ion, electron released from the metal acts on hexavalent chromium ion which is the principal component of the treatment solution on the ice surface of the metal to reduce it to trivalent chromium ion and, further as a result of increase in a pH of the solution thereabouts on account of decrease of a concentration of hydrogen ion attributed to the generation of hydrogen, a film of the aforesaid gluey complex compound is precipitated on the surface of the metal.

However, when these methods were applied to the treatment of iron and steel, a good film having corrosion resistance could not be obtained. The reason is considered to be attributable to the fact that iron and steel are rather diflicult to dissolve in the treatment solution in comparison wih magnesium, zinc, cadmium, and aluminum, whereby hexavalent chromium is not so sufficiently reduced to trivalent chromium as to form the film of the gluey complex compound on the surface of the metal.

Methods applicable to the treatment of iron and steel, which are today known as chromate treatment method, include a method which comprises treating iron and steel by dipping the same in a treatment solution, as the electrolyte, prepared by adding a small amount of reducing agent to an aqueous solution comprising principally of hexavalent chromium ion (Japanese patent publication NO. 3,219/60, U.S. Patent Nos. 2,768,103 and 2,768,104), a method which involves an electrolytical treatment of film by electrolyzing iron and steel used as cathodes in a treatment solution, as the electrolyte, prepared by adding a special additive to an aqueous solution comprising mainly of hexavalent chromium ion and utilizing reduction at the cathodes on electrolysis, and other like methods. In each of these methods, various additives to be added are intended to attain formation of film of good quality. There have been invented various additions, for example, an invention which intends to promote formation of film by adding boron or its salt (U.S. Patent No. 2,780,592), an invention which intends to promote formation of film by the addition of acid, salts or soluble complex salt capable of dissociating halogen ion (Japanese patent publication No. 14,363/ 64) inventions which comprises adding chemicals having phosphoric acid radical and utilizing a co-adherent effect of phosphoric acid film and chromate film (Japanese patent publication Nos. 3,514/55, 8,510/58 and 8,917/64), and an invention in which aromatic sulfonic acid is added to produce chelate compound with chromium, whereby formation of film is assisted by catalytic action of said compound (Japanese patent publication No. 8,207/ 60). However, for the purpose of utilizing the treated product in the wider fields of application, the further highly improved quality of the mm is required, especially in corrosion resistance thereof.

Accordingly, an object of this invention is to provide surface treatment method of metal product, which affords better corrosion resistance to the treated metal product than produced by the conventional methods.

Another object of this invention is to remarkably improve corrosion resistance of metal product, and at the same time is to form an electrolytically treated film on the surface of metal for the purpose of improving adhesitivity to an organic film of pigment and the like.

A further object of this invention is to provide a novel electrolyte to be used in the aforesaid process.

These and other objects of this invention will be apparent to those skilled in the art from the description of the invention hereunder described.

According to the process of the invention, it has been found that in a process for treating a metal product an excellent result can be obtained when a treatment solution prepared by adding a water-soluble organic compound containing a thio radical to an aqueous solution of chromium compound in which the chromium is predominantly present in the solution in the form of hexavalent chromium-containing ion is employed as an electrolyte, the metal product is used as a cathode and then the surface of the metal is subjected under appropriate electrolysis conditions to electrolytic treatment which provides corrosion resistance.

In the present invention, amongst the metal products to be treated, iron and steel are most effectively treated by the process thereof. However, the metal products are naturally not limited to iron and steel. Aluminum, copper and tin can also be treated as well. The aqueous solution of chromium compound, for example, includes an aqueous solution of chromium trioxide or salt of bichromic acid, and the like. The water-soluble organic compound containing a thio radical used in the present invention includes, for example, thioformamide, thiourea, thiosemicarbazide, thioglycol, thioacetate, and thiosalicylate. The amounts of these compounds to be added may be a small amount.

According to a preferred embodiment of this invention, the surface of metal to be treated is first cleaned by pretreatment such as degreasing, pickling and, waterwashing and then an electrolytic treatment is carried out using the metal to be treated as a cathode with the following composition of electrolyte and electrolysis conditions.

Chromium trioxide or bichromate Water soluble organic compound containing a thio radical Treatment temperature Treatment time Current density at the cathode More than a./dm.

0.52.0 g./l. 20-60 C. More than 1 second.

After completion of the electrolysis treatment, the product is water-washed and dried.

An object of the invention resides in the use of the Water-soluble organic compound containing a thio radical as described above. It is considered that a strong molecular adsorption effect of this water-soluble organic compound to the surface of the metal acts effectively on the formation of film upon the metal surface on electrolysis. That is, according to the process of the invention, numerous molecules of thio compound are absorbed onto the surface of metal in the treatment solution and, under such a surface condition of the cathode when hexavalent chromium ion is electrolytically reduced to trivalent chromium ion, the aforesaid gluey complex compound thereby produced grows as, a dense and hard film. Contrary to the fact that a film deposited according to conventional methods of this kind has been confirmed to be a non-crystalline substance, it has been clarified by electron diffraction that the film formed according to the process of the invention is deposited as a crystalline substance.

The detailed reason why the film grows as a dense and hard crystalline substance is still unknown. Of course, thickness of the film obtained is dominated by the electrolysis conditions and, in general, the larger the current density at the cathode and the longer the time of treatment, the thicker the film grows. However, a concentration of chromium compound in the electrolyte has scarcely relation with the thickness of the film obtained. Accordingly, as shown in the examples, such a film as having weight of 50-60 mg. per one dm. could be easily obtained by the treatment carried out for a short period of about ten seconds.

Thickness of films obtained by electrolytic treatment methods are thin in general. As can be seen in a process (Japanese patent publication No. 8,207/60) which involve addition of aromatic sulfonic acid, it has been said to be normal that weight of film is 4 mg. to about 8 mg. per one dm. According to the process of the invention, corrosion resistance of the treated product is remarkably improved, compared with those of conventional ones. In

the case of treatment effected on low carbon steel cold rolled sheet, the product does not become rusty when subjected to a salt water spray test in accordance with Japanese Industrial Standard Z237l, extending the spraying time until reaching 500 hours. Furthermore, in the conventional electrolytic treatment, color tone of treated film is delicately influenced by the electrolysis conditions such as a concentration of chromium compound in the electrolyte and temperature at the electrolyte, and the current density of cathode, and therefore the control of operating conditions for producing the product having a uniform color tone becomes diificult, especially in the continuous treatment of the steel strip. Contrary to the above, the color tone of film obtained in accordance with the process of the invention is scarcely dominated by the variations of the above-mentioned electrolysis conditions, whereby a film of beautiful pale blue having a strong water repellent property can be always obtained. The film obtained according to the invention is harder than those obtained by conventional methods, and therefore it is difficult to subject to scratching damage.

Furthermore, cross-cut adhesion test was conducted on sample on which melamine-alkyd paint has been coated in order to measure adhesivity of the film to paint, and as a result, there was obtained a favorable result wherein the number of squares remaining amounted to more than 95. As to adhesivity of the film to the surface of metal material, bending test, an Erichson test were conducted on various kinds of products to find that no peeling phenomenon of film in each test was observed at all.

The invention will be illustrated by the following preferred examples, but it should be noted that these examples are merely given as illustrative and not as limiting the scope of the invention.

EXAMPLE 1 Product to be treated Low carbon steel cold rolled sheet having 0.2 mm. thickness.

Chromium trioxide 50 g./l Thiourea 1 g./l. Treatment temperature 20 C. Treatment time 15 seconds.

Current density at the cathode 20 a./dm. Drying condition of film after water-washing At C. for 30 minutes.

The thus treated steel sheet had a beautiful pale blue color and the weight of film was found to be 60 mg. per one dm. No rust was generated thereon after salt water spray test (Japanese Industrial Standard Z237l) carried out for a period extending up to 500 hours.

EXAMPLE 2 Product to be treated Low carbon steel cold rolled sheet having 0.2 mm. thickness.

Chromium trioxide 100 g./l. Thioglycol 2 cc./l. Treatment temperature 15 C. Treatment time 10 seconds.

Current density at the cathode 10 a./dm. Drying condition of film after water-washing At 100 C. for 30 minutes.

The thus treated steel sheets had a beautiful pale blue color and the weight of film was found to be 3.8 mg. per one dm. No rust was observed thereon after salt water spray test (Japanese Industrial Standard Z-2371) carried out for a period extending up to hours.

EXAMPLE 3 Product to be treated Low carbon steel cold rolled sheet having thickness of 0.2 mm. Chromium trioxide 70 g./l. Thiosemicarbazide 2 g./l. Treatment temperature 30 C. Treatment time 5 seconds. Current density at the cathode 20 a./dm. Drying condition of film after water-washing At 100 C. for 30 minutes.

The thus treated steel sheet had a beautiful pale blue color and the weight of film was found to be 2.3 mg. per one dm. No rust was observed thereon after an outdoor exposure test carried out for a period of extending up to three months.

We claim:

1. A process for the surface treatment of iron and steel products which comprises electrolytically forming a protective film on the surface of said metal product by carrying out electrolysis in the presence of an electrolyte solution under the conditions suitable for forming a corrosion resistant film on the surface of the metal while using the metal product as a cathode, said electrolyte solution being an aqueous solution of a chromium compound in which the chromium is predominantly present in the solution in the form of hexavalent chromium-containing ions, said solution also containing a water-soluble organic compound containing a thio radical.

2. A process according to claim 1, wherein the aqueous solution of chromium compound is an aqueous solution of chromium trioxide or salt of bichromate.

3. A process according to claim 1, wherein the watersoluble organic compound is a member selected from the group consisting of thioformamide, thiourea, thiosemicarbazide, thioacetate and thiosalicylate.

4. A process according to claim 1, wherein a lead or graphite electrode is used as an anode.

5. A process for the surface treatment of iron or steel products which comprises'forming an electrolyte treatment solution by the addition of 0.5-2.0 g./l. of a watersoluble organic compound containing a thio radical to an aqueous solution containing 20-150 g./l. of chromium trioxide, and thereafter subjecting said product to electrolysis conditions while in contact with said electrolyte treatment solution, said product utilized as a cathode, said electrolysis conducted under a treatment temperature of 2060 C., a treatment time of at least 1 second, and a current density at the cathode of no less than 10 a./dm. whereby the dense protective film is formed on the surface of said product.

6. A process according to claim 5, wherein the watersoluble organic compound containing a thio radical is a member selected from the group consisting of thioformamide, thiourea, thiosemicarbazide, thioglycol, thioacetate and thiosalicylate.

7. A process according to claim 5, wherein the surface of the product to be treated is previously purified by such pretreatment as degreasing, pickling or water washing, etc.

References Cited UNITED STATES PATENTS 2,976,193 3/1961 Pimbley 148-621 2,898,250 8/1959 Pimbley 1486.21 2,746,915 5/1956 Giesher 204-56 FOREIGN PATENTS 745,042 2/ 1944 Germany.

HOWARD S. WILLIAMS, Primary Examiner R, ANDREWS, Assistant Examiner 

